Weather questions: How cold does it have to be to see your breath?

Robyn Graham sent in this photo of a rare frost in Taupaki in West Auckland this morning. Photo / Supplied

Kiwis earlier woke to frosts blanketing most of the country as temperatures dropped to -7C in parts of the South Island on what was one of the coldest mornings of the year.

Yet, not all places where frosts dusted the ground had official temperatures below zero. So just how cold does it need to be to have a frost?

And why is it usually colder just before dawn than in the middle of the night, and just how cold does it need to be to see your own breath?

With Kiwis freezing their breeches off across the country, we turned to National Institute of Water and Atmospheric Research meteorologist Chris Brandolini for answers to your burning weather questions.

How cold does it need to be to get a frost?

A young Brandolini remembers being back home in his upstairs apartment in the United States holding his thermometer as a 9-year-old and being puzzled.

He'd been told water freezes at 0C. Yet here he was upstairs with a thermometer showing 3C whereas on the ground he could see frozen puddles. How was it so?

Well cold air is heavy and it sinks, Brandolini told the Herald.

"That's why the term hot air balloon works, hot air rises, cold air sinks," he said.

"So the coldest air temperature is going to be found right at the surface."

Yet official temperatures are measured about 1.2m-1.3m above the surface, meaning official temperatures can be well above 0C even when frosts blanket the ground.

Highlighting this, Niwa recorded an official low temperature at Albany just before 6am today of 1.4C. The ground-level air temperature, however, was -4.4C.

At Mangere, near Auckland Airport, the official low at the same time was 2.7C. But the ground-level temperature was -1.3C.

Clear skies and light winds helped to reduce ground temperatures because this allowed more heat to escape into space and cold air to settle.

Why is it colder at dawn than in the middle of the night?

The sun heats the earth - that's how we get warm, Brandolino said.

"When the sun goes away, we lose our heat," he said.

"And generally you are going to be coldest within a half hour of sunrise because that is the point of maximum cooling."

Once the sun starts coming back up above the horizon, it's going to start heating the earth again.

How cold does it need to be to see your breath?

"If you're like me and eat a lot of garlic you can see your breath at 30C," Brandolino joked.

For all non-garlic eaters, there was no set temperature at which point your breath became visible because it also depended on factors like how much moisture is in the air.

Brandolino said it is almost like we are making a mini cloud when we breathe.

"Because that's all it is, it's condensation. When you exhale, you are exhaling carbon dioxide and water vapour - and that water vapour is condensing."

So if it's 10C and really dry, your breath will quickly be absorbed into the atmosphere.

If it's 10C and really damp, "you'll probably see a good puff of cloud coming from your breath".

"Think of the atmosphere like a sponge. If the sponge is super saturated it can't take any more moisture, but if the sponge is really dry it had the capacity to absorb that moisture."

However, this effect was more pronounced at marginally cold temperatures. If the air is already at freezing then you'll usually see a good puff of breath even if its damp.